Definitely Voldemort. He's the main baddie in the Harry Potter series. But in this fanfiction, there could be other villains too. Maybe some jealous Disney characters who don't like Harry getting all of Mickey's attention. Or some dark forces from the edges of both the wizarding and Disney worlds that team up with Voldemort to stop Harry and Mickey's new family dynamic.
The main antagonist would likely be Voldemort as he has been Harry's archenemy from the start. But in this unique fanfiction scenario, we could also consider Pete from the Mickey Mouse universe as a secondary antagonist. He might be jealous of Mickey's new adopted son and try to cause trouble for Harry. Also, some of Voldemort's Death Eaters could form an alliance with Pete to make things more difficult for Harry Potter.
In this fanfiction, it could start with Mickey Mouse accidentally stumbling upon the wizarding world. He comes across Harry Potter, who is in need of a proper family. Mickey decides to adopt him. As Harry grows up in this new environment, he combines the knowledge of magic from Hogwarts with the charm and cheer of the Disney world. He might even use some Disney - like magic to enhance his spells, creating a whole new kind of magic that no one in the wizarding world has ever seen before.
One possible main antagonist could be Voldemort still. Even though Harry is now under Sparda's protection, Voldemort would see Harry as a threat and would try to get rid of him. Another could be some of the demons from Sparda's world who see Harry as an intruder or a threat to their plans in the new world order that Sparda is trying to create with Harry by his side.
It could be another magical creature or group that is jealous of the power that the vixen holds and by extension, Harry as her adopted son. Maybe there is a group of dark wizards who want to harness the vixen's magic through Harry, and they become the antagonists as they try to capture him.
Voldemort or his followers could still be antagonists. They might not like the idea of Harry being under Euryale's protection as it could make him more powerful. Another possibility could be some creatures from Euryale's world who are against her taking in an outsider like Harry.
The Joker could be a main antagonist. He would be fascinated by Harry's magic and would try to use it for his own evil purposes or just to cause chaos. He might try to drive a wedge between Harry and Batman by using mind games.
It could be some jealous rivals of Arcuid. They might see Harry as an outsider and a threat, especially if they think Arcuid is giving Harry special treatment or powers. Maybe there are those who want to use Harry against Arcuid for their own gain.
A Muggle - born mouse. It could be a unique concept where a mouse from the Muggle world somehow ends up in the wizarding world. It would be interesting to see how it copes with all the magic and the rules of the wizarding society.
The Thalmor could be great antagonists. Their strict control over magic and their anti - human beliefs could put them at odds with Harry. They might see his different form of magic as a threat to their power in Skyrim.
One possible antagonist could be a corrupt wizard in a high - ranking position in the Ministry of Magic's financial department. This wizard might be trying to cover up his misdeeds and would see Harry as a threat to his scheme.
Scarecrow could also be a main antagonist. His fear - inducing toxins could be a real problem for Harry. Harry would have to use his magic to counter the effects of the fear toxin and also figure out a way to stop Scarecrow from spreading it all over Gotham.